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This guide covers Claude Code’s MCP client for local and remote servers. Its documented behavior can vary by Claude Code version; the linked MCP reference is the authority for current transport and configuration details.
1. Identify the server’s state and capture the useful error detail
In an active Claude Code session, enter /mcp. From a shell, use claude mcp list to see configured servers and claude mcp get <name> to inspect one server. A Failed to connect status means Claude Code could not connect to that server; it does not mean the listing command failed.
- Needs authentication: investigate the remote server’s sign-in or configured credentials.
- Pending approval: resolve workspace trust and approve the project server.
- Rejected: check whether the server is disabled by settings.
- Disabled: enable it in
/mcpif you intend to use it. - Failed to connect: use the error details to decide whether to check configuration, process launch, authentication, or the network.
- Connected, but a tool is missing: check tool discovery and the server’s available tool list before assuming the connection is broken.
Connection details may include an HTTP status and a message returned by the server. Claude Code redacts credential-like strings and may avoid displaying a fully expanded URL that could contain secrets. Do not post unredacted configuration, authorization headers, tokens, or credential-bearing URLs. The Troubleshooting guide documents /doctor for a running session and claude doctor when Claude Code will not start; neither replaces inspecting the specific MCP entry and server logs. For additional client detail, use claude --debug, claude --debug-file <path>, or claude --verbose as described in the CLI reference.
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2. Check that the configured transport matches the server
The transport determines whether Claude Code launches a local process or contacts a remote endpoint. Anthropic’s Claude Code MCP documentation recommends HTTP for remote MCP servers where available.
| Transport | Use it when | Check first |
|---|---|---|
| Remote HTTP | The server exposes a remote HTTP MCP endpoint. | Endpoint URL and declared type, credentials or OAuth, proxy, firewall, and TLS. |
| Remote SSE | The server exposes only SSE, or compatibility with the server or Claude Code version requires it. | Whether the server supports HTTP instead; SSE is deprecated in the documentation, which describes HTTP-first fallback on newer releases. |
| Local stdio | The server is a local process, script, package, or tool that needs access to the machine. | Executable, arguments, environment variables, shell quoting, process output, and operating-system-specific launch requirements. |
| Remote WebSocket | The server offers a WebSocket endpoint supported by Claude Code. | A wss:// endpoint and header-based authentication. The CLI --transport option does not accept ws; configure WebSocket through JSON configuration or /mcp. |
A common mismatch is a remote configuration entry containing url but no type: Claude Code interprets that entry as stdio. Set the type to match the remote endpoint—http, sse, or ws as appropriate. To add a remote HTTP server from the CLI, the documented form is claude mcp add --transport http <name> <url>. For a local command, put the launch command after --; specify any required --env values before that separator. When using claude mcp add-json, check that shell quoting preserves the JSON structure.
3. Fix project trust, approval, and disabled-server states
A server declared in a project’s .mcp.json may remain pending until Claude Code trusts the workspace and you approve the server. Open Claude Code in the project, accept the workspace trust prompt if appropriate, then review and approve the server interactively. A cloned repository cannot approve its own project servers through checked-in settings while the folder remains untrusted.
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If /mcp shows the server as disabled, enable it there. If it is rejected, inspect the disabledMcpjsonServers setting. These states are approval or configuration decisions, not evidence that the endpoint is unreachable.
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4. Diagnose remote authentication and HTTP failures
For an OAuth-protected remote server, start its sign-in flow through /mcp or, where appropriate, claude mcp login <name>. If the server reports 401 or 403, check that the account or credential has the required access and that the configured header or helper supplies the expected value. An HTTP response with a status and server message is different from an endpoint Claude Code cannot reach; use that distinction to choose the next check.
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For a custom authentication helper, the MCP reference requires it to emit a JSON object whose values are strings and gives it a 10-second execution limit. On a tool call that receives 401 or 403, Claude Code reruns the helper, reconnects, and retries once. Avoid placing secrets in screenshots, support posts, or shell commands likely to be saved in history.
Environment-variable expansion can also affect credentials. In .mcp.json, ${VAR} expands a variable and ${VAR:-default} supplies a fallback. An unset ordinary variable without a default is reported as missing and can remain literal in the configuration. Credential variables in remote URLs and headers are treated differently: some may resolve as empty to prevent a project configuration from forwarding Claude or provider credentials to a named server. Consequently, a 401 can result from that variable policy rather than a server outage. Review the MCP environment-variable and authentication guidance without sharing the expanded secret.
5. Troubleshoot a local stdio process that will not start or closes
A stdio connection depends on Claude Code being able to launch the configured command in its own environment. Check these items in order:
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- Executable: confirm the configured executable exists and is available to the environment running Claude Code.
- Arguments: check that arguments follow the executable in the intended order and that shell quoting has not changed them.
- Environment: pass any required variables to the server process, using the configured environment entries or CLI
--envoptions. - Process output: inspect the server’s stderr or logs for its actual startup error or reason for exiting.
- Platform launch behavior: adapt launch settings copied from another MCP client to Claude Code’s configuration. On native Windows, the official reference documents an
npxlaunch wrapper usingcmd /c; invokingnpxdirectly in that environment can result in a connection-closed error.
Connection closed is a symptom, not a single diagnosis. For stdio, investigate whether the process launched and stayed alive; for a remote server, check the endpoint, transport, authentication, and network route instead. Consult the version- and platform-specific examples in the MCP reference rather than copying a command intended for another client.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.6. Resolve a connected server with a missing tool or a tool-call error
First inspect the server and its tool list in /mcp. A missing tool does not by itself prove the server configuration or tool name is wrong. Remote HTTP and SSE servers can use cached discovery, and deferred discovery can mean Claude Code has a prior tool list but will connect when a tool is first used. During an initial connection, a call may wait up to 10 seconds; if the server has not connected or is already retrying, the call can fail with No such tool available. Check the connection state again after it changes, then verify the tool’s name and availability with the server.
If the server is connected and the failure happens only after invocation, compare the client’s returned error with server-side behavior and logs. This is distinct from a connection failure. Also distinguish an oversized result from either problem: the current MCP reference lists a 10,000-token warning threshold and a 25,000-token default maximum for applicable MCP tool results. The maximum can be adjusted with MAX_MCP_OUTPUT_TOKENS; raising it addresses output handling, not a failed connection.
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7. Check proxy, certificates, and network access for remote servers
When the endpoint and credentials look correct but the remote server remains unreachable, verify the route from the machine and session running Claude Code. The enterprise network configuration guide documents HTTPS_PROXY and HTTP_PROXY for proxies, NODE_EXTRA_CA_CERTS for custom CA trust, and client certificate and key variables for mutual TLS. It also documents NO_PROXY behavior.
Confirm which values Claude Code loaded using debug logs and /status. A setting can be accepted syntactically and still fail on a later connection. Firewall rules, proxy requirements, and allowlists depend on the organization’s network and the server; ask the network administrator to verify the required route if you cannot confirm it locally.
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